The present invention relates to a method of treating an electroless plating bath, particularly an electroless nickel plating bath, containing hypophosphorous acid and/or a hypophosphite such as sodium hypophosphite as a reducing agent, which is capable of effectively treating nickel phosphite produced when the plating bath is regenerated and repeatedly using the plating bath without wastage.
Electroless plating, particularly electroless nickel plating (typically, electroless Ni--P plating) using sodium hypophosphite as a reducing agent has been used extensively in a variety of industrial fields. In the electroless plating of this type, metal ions are reduced by hypophosphite ions to be thus deposited on a substance to be plated.
Accordingly, with progress of plating, metal ions and hypophosphite ions in the plating bath are consumed, so that the deposition rate becomes smaller and/or the composition of deposit or plating film (for example, alloy contents of Ni and P for a Ni--P plating film) is changed. Therefore, plating is generally performed by compensating for the consumed metal ions and phosphite ions to keep concentrations thereof at the initial or predetermined values.
In the electroless plating, however, when metal ions are reduced by hypophosphite ions, the hypophosphite ions are simultaneously oxidized to produce phosphite ions (H.sub.2 PO.sub.3.sup.-), and the phosphite ions thus produced are gradually accumulated in the plating bath.
The phosphite ions (H.sub.2 PO.sub.3.sup.-) in a small amount little affect plating; however, when they are accumulated in a large amount, for example, in the range of 50-100 g/l or more, particularly, in higher than 150 g/l in concentration, they possibly exert adverse effect on plating. For example, by the effect of the phosphite ions accumulated in a large amount, the bonding strength between a plating film and a substrate may be weakened; the uniformity and corrosion resistance of the plating film may be degraded due to codeposition of the phosphite ions in the plating film; and in the worst case, the plating bath may be decomposed.
For this reason, in electroless Ni--P plating, an electroless Ni--P plating bath is generally discarded after used 3-10 turns. One turn is equivalent to a state in which the whole amount of nickel ions contained in the initial electroless Ni--P plating bath is consumed or deposited. For example, assuming that 6 g/l of nickel ions in concentration exists in a fresh solution or the initial electroless Ni--P plating bath, a state in which the nickel ions in an amount of 6 g/l are consumed or deposited is taken as one turn. Accordingly, 3-10 turns mean that the nickel ions in an amount of 18-60 g/l are consumed or deposited.
In addition to the problem that the service life of an electroless Ni--P plating bath is relatively short as described above, the Ni--P electroless plating has another industrial problem on waste water treatment accompanied by discard of the electroless Ni--P plating bath. To solve these problems, there have been proposed various methods of regenerating an electroless Ni--P plating bath. The related art regenerating methods, however, have been not extensively used because of disadvantages, for example, the complicated treatment, inclusion of impurity ions harmful to plating in the regenerated plating bath, or expensive system.
In view of the foregoing, the present inventors have proposed a method for regenerating an electroless nickel plating bath in Japanese Patent Laid-open No. Hei 5-247660. This method is characterized by regenerating an aged electroless nickel plating bath containing a water-soluble nickel salt, a complexing agent, and hypophosphorous acid or a hypophosphite as a reducing agent, and also containing 100 g/l or more of phosphite ions produced by oxidation of the hypophosphorous acid or hypophosphite with progress of plating. Specifically, the water-soluble nickel salt is added to the above aged plating bath at a ratio of 0.5 mol or more with respect to 1 mol of the above phosphite ions to produce and precipitate nickel phosphite, and the nickel phosphite thus precipitated is separated from the plating bath.
The above method, however, fails to examine the effective use of the nickel phosphite separated from the plating bath, and particularly to propose the effective recycle of the electroless plating bath.